How could the government know about a chemical attack before it wreaks havoc? By smelling it.

But the problem is, to detect an abnormal stench, the government first needs to know the city’s normal aroma, to have an idea of its “chemical profile.” To that effect, DARPA just released a solicitation looking for suggestions on how to best build chemical composition maps of major United States cities. Spencer Ackerman over at Wired’s Danger Room t0ok a look at the solicitation and explained what DARPA is looking for:

The data Darpa wants collected will include “chemical, meteorological and topographical data” from at least 10 “local urban sources,” including “residences, gasoline stations, restaurants and dry cleaning stores that have particular patterns of emissions throughout the day.”

Then, subsequent chemical readings from the area could be compared to the “map” to check for abnormal chemicals in the air. Since many chemicals that can be used in a terrorist attacks are normally found around our cities, it’s difficult to just screen for them without having an idea of their baseline levels, explains Wired:

In theory, chemical attacks can be detected before they happen. Even trace amounts of chemicals give off specific signatures that tools like sorbent tube samplers can register. But in order to figure out if dangerous chemicals are stockpiled somewhere or are floating through the air, the government’s going to have to know the baseline level for those chemicals wafting near your trash receptacle.

DARPA is looking for proposals on how best to collect data while spending less than 30 minutes doing it. They also want to collect information on the two-day fluctuations in chemicals, and take readings at different times of the year, says Wired.

Then they’ll adjust for atmospheric and environmental variables like wind speed, humidity and time of day–when, say, the dry cleaners’ is open to spew perchloroethylene vapor into the air–to account for the impact on chemical potency. They’ll use that data to “predict concentrations down to trace gas concentrations of 10 parts per trillion” across a whole city.